1. Field of the Invention
The present invention relates to a process for preparation of ceramics wherein powder grains are oriented in a definite direction.
More specifically, the present invention relates to a process for preparation of a .beta.-or .beta."-alumina solid electrolyte which is used in sodium-sulfur (Na-S) cells whose utilization for electric cars and for smoothing electric power load is expected and which is minute and has a high conductivity of sodium ion and an excellent mechanical strength; and a magnetic anisotropic polycrystalline sintered barium or strontium ferrite magnet for generating a strong magnetic field (hereinafter referred to as anisotropic ferrite magnet).
2. Related Art Statement
.beta.-Alumina is a substance generally represented by Na.sub.2 O.multidot.11Al.sub.2 O.sub.3,belongs to the hexagonal system and has a structure of repeated one spinel block. .beta.-Alumina is called a two-dimensional ion electroconductor since Na.sup.+ ion can move only in the C-face perpendicular to the C-axis.
The electric resistance (specific resistance) in the C-face of .beta.-alumina single crystal is excellent and 8.OMEGA..multidot.cm at 300.degree. C., and if a cylindrical or tabular material consisting of single crystals whose C-face is arranged to the direction to be electrified is used, a high electroconductivity can be obtained. However, cleavage at the C-face is liable to occur in .beta.-alumina single crystal and thus there is a problem in view of mechanical strength and growth of single crystal is technically difficult, and thus it is difficult to practically use such a material.
Although it can be attained by preparing a polycrystalline body consisting of small crystal grains to enhance mechanical strength, in a polycrystalline body electric resistance is large at the interface of crystal grains and since the azimuth of each grain is irregular the direction of movement of Na.sup.+ ions is different. Thus, electric current follow a zigzag course and electric resistance increases. For example, the specific resistance at 300.degree. C. of a polycrystalline body prepared by sintering .beta.-alumina is 20 to 100.OMEGA..multidot.cm and fairly higher than that of single crystal, and thus in order to enhance the performance of Na-S cells it is necessary to reduce specific resistance.
.beta."-Alumina is a substance represented by Na.sub.2 O.multidot.5Al.sub.2 O.sub.3, and the specific resistance of single crystalline body is 0.5 to 1 .OMEGA..multidot.cm and the specific resistance of sintered polycrystalline body is on the order of 5 to 8.OMEGA..multidot.cm. Although the specific resistance of .beta."-alumina is fairly improved compared to that of .beta.-alumina, it is necessary to make specific resistance further decrease for further enhancement of the performance of Na-S cells.
Usual method for reducing the resistance of .beta.-or .beta."-alumina solid electrolyte is a method of thinning the thickness of the solid electrolyte which functions as a passage of Na.sup.+ ions. Although solid electrolyte having a thickness of about 1mm is now prepared, it is difficult in view of mechanical strength to prepare solid electrolyte having a thickness of 1mm or less and the reduction of resistance is hardly expected by a method of thinning the thickness of the solid electrolyte.
M type hexagonal system ferrite such as barium ferrite or strontium ferrite is powder consisting of hexagonal tabular fine grains which have C-axis as an axis of easy magnetization and have a developed C-face. Although anisotropic ferrite magnets wherein this powder is oriented are widely used as a permanent magnet for magnetic circuit, in accordance with recent request for miniaturization and/or performance enhancement of speakers, small-sized motors, etc., enhancement of the magnetic characteristics of ferrite magnets is strongly desired.
Particularly, in order to generate a strong magnetic field in magnetic circuits using an anisotropic ferrite magnet, high residual magnetic flux density (Br) and high coercive force (Hc) are desired.
Barium ferrite or strontium ferrite is usually used as an anisotropic ferrite magnet, and its residual magnetic flux density (Br) is on the order of 4.0 to 4.4 KG and its coercive force (Hc) is on the order of 2.8 to 3.3 KOe. Although in order to make the above desire possible the improvement (magnetic field press molding) of ferrite composition and/or orientation method of ferrite grains has been carried out, satisfactory magnetic characteristics have not been obtained.